The drive train in certain rotary wing aircraft, such as helicopters, may be powered by a turboshaft engine that includes a gas generator and a free turbine, for example, which can be mechanically coupled to the main and tail rotors of the aircraft, respectively. If weakly damped, the rotor shaft in such drive trains may exhibit moderate to strong resonance at certain frequencies, for example, in or around the 2-8 Hertz (Hz) range. While some damping of the free rotor resonance modes may be provided by aerodynamic drag on rotor blades, gearbox reductions, and/or mechanical losses, without provision of additional damping, such free rotors may experience sizable speed oscillations at or near their resonant frequency(ies).
Disturbances such as exposure to a large wind gust or a sudden change in pitch may cause free rotors to “ring”, i.e., develop speed oscillations that can last for seconds in some cases. Sufficiently large and long lasting speed oscillations may adversely affect stability and/or speed control in the aircraft.
One approach to regulating rotor shaft speed in certain turboshaft engines is to employ a closed loop feedback control system that regulates fuel supply to the engine's gas generator based on the sensed rotor speed of the free turbine or coupled rotor shaft. For example, such control systems may utilize a free turbine speed controller responsive to an error signal generated between sensed and commanded rotor speed. However, to avoid potentially de-stabilizing resonant frequency excitation within the free turbine control path, certain gas generator fuel control systems incorporate a passive filter in order to suppress and prevent the free turbine speed controller from acting on components of the sensed rotor speed that may be attributable to rotor resonance. Because of such signal suppression, many gas generator fuel control systems of this general configuration will not enhance, and in fact may tend to decrease, the amount of inherent damping that might otherwise act on a free turbine or coupled rotor shaft.